This invention relates generally to surface-modified particles of materials such as aluminum oxyhydroxides, iron oxyhydroxides, and clays, and methods for preparing the same.
Particulate fillers have long been known to impart desirable properties to a variety of polymeric materials. For example, mica increases the stiffness of phenol-formaldehyde plastics (A. King “Application of Fillers” in Plasticizers, Stabilizers, and Fillers, P. D. Ritchie ed, Iliffe Books, London, 1972.) Plate-like fillers have been known to improve the barrier properties of their composites (A. A. Gusev and H. R. Lusti, “Rational Design of Nanocomposites for Barrier Applications”, Advanced Materials, 2001, Vol. 13(21), 1641–1643).
In many cases, it is advantageous to provide inorganic particles with an organic surface modification. Modifying the surface of particles that are added to a polymer matrix to form a composite can improve the wetting of the particles by the matrix and improve the dispersion of the particles in the matrix, thereby improving such properties of the composites as strength, toughness, and the ability to act as a barrier. Surface modifications can also improve the adhesion between the particles and the polymer, thereby improving the load transfer and the mechanical properties of the composite. For example, U.S. Pat. No. 4,091,164 teaches the modification of kaolin clays by mixing the clay particles with block copolymers of ethylene oxide and propylene oxide and then melting the polymers on the clay particles. Surface modifications have also provided particulate fillers the ability to bond with a matrix, as is described in e.g. U.S. Pat. No. 3,901,845, which teaches coupling of a mineral filler with a nylon matrix by an aromatic compound having a carboxyl group and a hydroxyl or amine group. PCT application WO 00/09578 also teaches the surface modification of a filler followed by coupling of the filler to an organic matrix.
Polymer-clay composites have received much attention in the past five years (LeBaron, Wang and Pinnavaia, Applied Clay Science, 1999, 15, 11–29), and most of this work has focused on alkylammonium-exchanged smectite clays. Addition of surface modified clays to polymers improves the properties of the polymer. For example, by adding a few percent loading of clay to nylon-6 increases the heat distortion temperature by 80° C. This increase makes structural applications possible under conditions where the pristine polymer would fail (deform). In another example, similar low loading levels of surface-modified clay increase the toughness and the tensile strength of thermoset materials such as elastomeric epoxies and polyurethanes. Furthermore, in glassy epoxy composites, clays greatly improve the yield strength and modulus under compression.
Although adding surface modified particles is an extremely important way of improving the properties of composite materials, the chemistries available for modifying a particle's surface in a single step are limited. Large carboxylic acids do not readily react with the surface of aluminum oxyhydroxides such as boehmite due to conformational and steric limitations. Likewise, large quaternary ammonium compounds diffuse very slowly into clay galleries, limiting the rate of production at which some surface-modified clays can be produced. Moreover, certain functional groups cannot be introduced in a one step modification because they undergo undesired side reactions.
U.S. Pat. No. 5,593,781 (Nass, et al.) describes surface modification of ceramic powders of nanometer size particles with small molecular weight organic compounds in a one-step process by dispersing the ceramic powder in water or an organic solvent and adding the low molecular weight organic compound.
Apblett et al. [Mat. Res. Symp. Proc. Vol. 249 1992] describe the formation of carboxy substituted particles from the reaction of pseudoboehmite and carboxylic acids in a one-step process.
Landry et al. [J. Mater. Chem. 1995, 5(2), 331–341] describe the reaction of [Al(O)(OH)]n, with carboxylic acids to form [Al(O)x,(OH)y(O2CR)]n where R=C1–C13 and 2x+y+z=3 using a one-step reaction.
U.S. Pat. No. 6,369,183 (Apr. 9, 2002) describes thermoset polymer networks formed from amine, hydroxyl, acrylic and vinyl substituted carboxylate-modified boehmite with low molecular weight polymer precursors.
U.S. Pat. No. 6,322,890 (Barron and Obrey) describes supra-molecular alkylalumoxanes comprising a) an aluminum-oxide nanoparticle, b) a linkage unit, and c) an alkylalumoxane. The alkylalumoxanes used in Barron and Obrey are distinct from the carboxylato-alumoxanes of Landry et al. and serve as co-catalyst for alkene polymerizations (e.g. methylalumoxane (MAO)) by an organometallic reaction. Thus, Barron and Obrey teach the use of an aluminum oxide nanoparticle as a base on which to attach additional alumoxane units. Significantly, Barron and Obrey link these alumoxanes to the particle surface with organometallic bonds.
In U.S. Pat. No. 4,349,389, inorganic metal-containing pigments (particularly titanium dioxide) are made dispersible in thermoplastics and paints by first rendering the pigment hydrophobic by coating it with an alkylbenzene sulfonic acid, then coating it further with a thermoplastic polymer. Significantly, no covalent bond is created between the first surface-modifying molecules and the subsequent molecules that are simply adsorbed onto the modified surface.
U.S. Pat. No. 4,764,495 (Rice), describes surface modification of clay minerals by hydrogenation followed by reaction with molecules containing unsaturated carbon-carbon bonds. This is a two-step modification in which the first step entails reacting the particle with an inorganic gas rather than an organic compound. U.S. Pat. No. 4,900,767, teaches a method of modifying clays by contacting the clay with an organic monomer or prepolymer which is subsequently polymerized in the presence of carbon monoxide.
U.S. Pat. No. 5,814,407 (Richard, Vaslin and Larpent) describe forming latex or silica particles having N-alkylglycosyl groups on the surface by reacting an amine, thiol or phenol-functionalized latex particle or silanol-functionalized silica particle with an N-alkylacrylamidoglycosyl group. The objective of this patent is to produce materials that can find applications as detection agents in biology. The patent does not describe methods for two-step surface modifications to silica; silanol groups are inherent in the surface of silica and the origin of the surface-bound amine, thiol or phenol groups is unclear. At any rate, the present invention relates neither to —O—Si—anchor groups nor to silica surfaces.
U.S. Pat. No. 6,224,846 (Hurlburt and Plummer) describes the formation of a modified boehmite alumina by reaction of boehmite with a sulfonic acid at temperatures between 90° C. and 300° C. and preferably between 150° C. and 250° C. However, the patent does not teach the reaction of a sulfonic acid group with boehmite followed by reaction of another organic group with the attached sulfonic acid group.
There is a need in the art for surface-modified particles and multi-step methods for preparing the same.